<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        // 手写一个简单的Ajax

        // 结合Promise
        function Ajax(url) {
            const p = new Promise((resolve, reject) => {
                const xhr = new XMLHttpRequest();
                xhr.open('GET', url, true);
                xhr.onreadystatechange = function () {
                    if (xhr.readyState === 4) {
                        if (xhr.status === 200) {
                            resolve(
                                JSON.parse(xhr.responseText)
                            );
                        } else if (xhr.status === 404) {
                            reject(new Error('404 not found'))
                        }
                    }

                }
                xhr.send(null);
            });
            return p;

        }
        const url = './data/test.json';
        Ajax(url).then(res => console.log(res)).catch(err => console.err(err))

    </script>
</body>

</html>